software | Process optimisation
Figure 1: Temperature distribution on the cavity surface at the beginning (top) and end (bottom) of cycles 1, 2 and 10
value of 20°C. However, once the hot thermoplastic melt starts to fl ow in the cavity, it begins to heat the mould steel. Some of this heat is dissipated over several moulding cycles, but some is retained. Eventu- ally, the mould will reach a quasi-stable thermal state. The theoretic solidifi cation time determined over the
cross section of the part in the initial analysis can now be compared with the time actually required for the ‘virtual’ real mould. As the ‘virtual’ mould temperature is actually some way away from the homogenous 20°C assumed in the fi rst simulation, the actual cooling time of the part is longer. If the mould temperature was at a uniform 20°C the part would solidify after 30s. However, as the temperature in some regions of the cavity reaches as high as 50°C, the virtual molding analysis shows that a considerable volume of material remains liquid after 30s.
Armed with this ‘real’ process data it is possible to
test alternative process optimisation options in the virtual environment. In this example, the fi rst modifi ca- tion is to place the water cooling channels closer to the cavity walls. The second option is to use a more costly high conductivity steel for the mould cavities. The resulting data shows while the high conductivity steel increases the mould cost by 5%, it reduces the cycle time by 25%. Part removal time also has an impact on cycle time
– for every second the mould is open it is dissipating heat into the mould shop environment. The graphs in Figure 2 show the effect that part removal times of 3s and 8s have on the temperature of the mould and the solidifi cation time of the part. It this example it can be seen that reducing the part removal time by 5s from 8s to 3s results in a signifi cantly higher mould tempera- ture. However, this higher mould temperature only increases the solidifi cation time by 1s, resulting in a net cycle time saving of 4s through faster part removal. One company to successfully apply the Virtual
62 INJECTION WORLD | January/February 2015
Molding approach is US-based Kalypso Ultra Technolo- gies, which used the software to develop the moulds for production of a 100mm diameter thick-wall motor mounting in 30% glass reinforced PA6,6. The combina- tion of the material, the complex geometry, and tolerances around each mounting diameter of +0.07/- 0.00mm made it a challenging part that the company did not want to approach using ‘trial and error’. Instead, it used a 20 cycle Virtual Molding analysis where the predicted process parameters revealed hot spots on the mould once steady-state conditions were obtained. Hot spots result in variations in crystallinity in semi-crystalline polymers such as PA6,6, leading to non-uniform shrinkage and potential distortion. Kalypso used the SigmaSoft software to evaluate the use of conformally cooled mould cores, solving both the temperature issues and reducing cycle time.
www.sigmasoft.de
Figure 2 – Shortening part removal time by 5s results in a net cycle reduction of 4s as less heat is dissipated from the open mould
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